1. Field of the Invention
The present invention relates to an ink jet head for performing record by discharging ink to a recording medium.
2. Description of Related Art
In recent years, an ink jet recording apparatus has been widely used especially as an output device of a computer because a high definition character and an image can easily be obtained by means of the ink jet recording apparatus. Inter alia, the bubble jet system for discharging ink from nozzles by means of a sudden pressure change produced by boiling the ink in the nozzle rapidly has become the main stream of the ink jet recording apparatus since many nozzles can easily be arranged in a high density in a simple configuration by the bubble jet system.
Moreover, as the ink jet recording apparatus has been widely spread in recent years, demands for the performances of the ink jet recording apparatus, especially for the image quality thereof and the recording speed thereof, have been increased. For the improvement of the image quality, it is important to reduce the diameters of dots recorded on a recording medium (especially on a sheet of recording paper). The demand is remarkable in case of the record of images represented by a photographic image in comparison with character documents. For example, the resolution necessary for obtaining the beauty of characters or for resolving small characters in the record of a character document is within a range from 600 dpi to 1200 dpi, and it is consequently enough for obtaining the resolution that the diameters of dots of liquid droplets to be discharged are within a range from about 80 μm to 90 μm (about 30 pl in case of being expressed by the volume).
On the other hand, in case of performing image record, the resolution, for example, for expressing smooth gradation equivalent to that of a film photo is required to be within a range from 1200 dpi to 2400 dpi. If the diameters of dots of liquid droplets to be discharged are 40 μm (about 4 pl in case of being expressed by the volume) in case of record with the resolution mentioned above, it is required to use two kinds of inks having the densities of dyes different from each other by the degree from about ¼ to ⅙ properly according to the densities of images. If the diameters of dots of liquid droplets to be discharged are made to be as small as about 20 μm (0.5 pl in case of being expressed by the volume), both of the requirements for density in a high density part and for smoothness in a low density part can be satisfied without any conflict by means of a kind of ink of a single density. As described above, it is essential for obtaining an image quality equivalent to a film photo to achieve the reduction in size of the liquid droplets to be discharged.
An ink jet head configured to discharge small liquid droplets is required to increase the number of times of discharging liquid droplets per a unit time. Consequently, the amount of current flowing a heat generating member increases, which in turn generates a large voltage drop at a parasitic resistance in a wiring section up to the heat generating member. Thus, the ink jet head has a problem of a decrease of its discharge efficiency. For preventing the decrease of the discharge efficiency, a method for decreasing current values by increasing the resistance value of the heat generating member is effective. It can be considered to increase the resistance value of the material of the heat generating member as means for increasing the resistance. However, there is a limit in increasing the resistance value by changing the material of the heat generating member. Besides, if a new material is used, a necessity to examine the new material fully whether there is some functional problem or not is generated. The change of the material of the heat generating member is difficult to realize. Accordingly, the increase of the resistance can be realized by dividing the heat generating member into a plurality of pieces to be connected in series and by arranging the pieces in an ink flow passage.
However, it was found that a new problem is produced as another problem in case of arranging the heat generating member after dividing it into a plurality of pieces.
Since the structure of an ink jet head is fine, as shown in FIGS. 10A and 10B, there is a case where the center of a heat generating member 1102 provided on a substrate 1101 and the center of a discharge port 1104 provided on a flow passage forming member 1103 are shifted from each other owing to the dispersion generated in a manufacturing process. A reference numeral 1105 designates an ink flow passage, and a reference numeral 1106 designates an ink feed passage.